1. Field of the Invention
This invention relates to an optical recording method and apparatus for recording data information in an optical recording medium, and an optical reading and apparatus for reading data,information from an optical recording medium.
2. Description of Related Art
Phase change type and magneto-optical type rewritable optical disc apparatus have been widely used. The recording density of these optical disc apparatus is one order or more higher than that of general magnetic disc apparatus, however insufficient::for digital recording of the video image. To increase the recording density, it is necessary that a beam spot diameter should be reduced so that the distance between adjacent tracks or adjacent pits is reduced. One of such technologies which has been already commercialized is DVD-ROM. A DVD-ROM stores 4.7 GBytes data on one side having a diameter of 12 cm. The recording density of a phase change type writable and erasable DVD-RAM is as high as 5.2 GBytes on both side of a disc having a diameter of 12 cm. This data quantity which is equivalent to four or more CD-ROM""s, and to 1900 floppy discs can be written/read.
As described hereinabove, the recording density of the optical disc has been increased year by year. However, on the other hand, because data are recorded on in the plane of the above-mentioned optical disc, the attainable recording density is limited by the optical diffraction ultimate and the actual recording density approaches the physical limit (5 GBytes/in2) To increase the recording density more, it is necessary that three dimensional (volume type) recording including in depth direction or multi-level recording in a single recording pit is applied.
Photo-refractive material has been known as the above-mentioned volume type optical recording medium. Because this material causes change in refractive index when the material absorbs a relatively weak light, and it is possible to use for information recording in the form of photo-induced refractive index change. Therefore, the material can be used for volume multiple holographic recording, which has a large capacity. For example, LiNbO3 is used as the recording medium of a digital holographic memory described in xe2x80x9cVolume Holographic Storage and Retrieval of Digital Dataxe2x80x9d SCIENCE JOURNAL, Vol. 265, pp. 749-752 (1994). This optical system is structured as described hereunder. First, a light emitted from a light source is divided into two light waves by a beam splitter. A light which passes thorough the beam splitter is converted to a collimated light having a wide aperture by a lens and entered to a spatial light modulator. The spatial light modulator which is controlled by a computer generates signal lights having two dimensional intensity distribution. The signal light is focused onto a recording medium (LiNbO3) by a lens. On the other hand, a light reflected by the beam splitter is reflected by a mirror andand entered to the above-mentioned recording medium. This light is used as a reference light. The signal light and reference lights are incident simultaneously upon the recording medium to record data holographically. To read out the hologram, only the reference light is incident upon a recording medium, the reference light is diffracted onto the optical path of the signal light as if the signal light would pass through the recording medium, and the light is imaged on a camera by a lens.
This system uses a differential code method. In this method, a pair of pixels are used, for example, 0 is represented by xe2x80x9cdark brightxe2x80x9d and 1 is represented by xe2x80x9cbright darkxe2x80x9d. Because the number of bright and dark is the same always, and the light intensity of object light which has been generated by the spatial light modulator is constant. Therefore, it is not necessary to adjust the reference light intensity every page. In reproduction of a hologram, the light intensity is apt to fluctuate and it is difficult to define the white black level uniformly, however in the differential code method, only the edge may be read and therefore noise hardly affects the reproduced signals.
On the other hand, in the field of the in-plane recording method, a technology for recording multi-level data in a single recording pit has been proposed. The principle of opto-magnetic recording is based on reading of polarization angle change of a reflected light due to electro-optical Kerr effect which occurs in the recording area. In spite of various devices, the polarization angle change obtained by Kerr effect remains in a small change as small as 1 degree. Therefore it is difficult to convert the data into multi-level within small change, and the device to widen the dynamic range of a signal has been tried. For example, Japanese Published Unexamined Patent Application No. Sho 64-17248 discloses a recording medium having a multiple recording films different in Curie temperature as a means to realize the multi-level recording. Furthermore, Japanese Published Unexamined Patent Application No. Hei 4-38720 discloses a method for obtaining multiple value by changing the inclination of an ellipse recording pit which is used as a recording pit as a means to record/reproduce multi-level in an optical disc.
However, the above-mentioned digital holographic memory is disadvantageous in the problem described hereunder. Usually a signal light having data information and reference light which interferes with the signal light are necessary. It is required that the signal light and reference light should be generated by separating the same light source and the optical path difference between these two light waves is within the coherent length of the laser. A laser diode could be the light source as one method, but because the coherent length of a laser diode is very short, high accuracy is required for the alignment of the optical system. Furthermore, because the signal light and reference light pass independent optical paths and are affected by outside light and noise of the optical system respectively, and these factors causes deterioration of a reproduced image. Furthermore, because independent optical paths are necessary respectively for the signal light and reference light, the system cannot be made compact disadvantageously.
The digital holographic memory is expected to be large capacity duel to volume multiple recording, however there is an inevitable problem associated with the principle of the hologram multiple recording. The problem is that the diffraction :efficiency of the individual reproduced image decreases in proportion to the square of the degree of multiplex. Therefore, if the efficiency of 100% is obtained theoretically, the 10 multiple recording results in the efficiency of 1%. Furthermore, because scattering noise is independent of the degree of multiplex, S/N decreases significantly. If the digital holographic memory is used as a file memory for a computer, the digital holographic memory cannot satisfies the required bit error rate due to reduced S/N, therefore cannot be used for high density recording currently. Therefore, it is expected to develop a memory with high S/N used while high speed transfer due to parallel recording/reading of the holographic memory is maintained.
On the other hand, the in-plane recording method in which multi-level data is recorded in a single recording pit is also disadvantageous in that the transfer speed is slower than that of holographic memory, in which a plurality of pits arranged two-dimensionally are read out at a time, because data is read from pits one by one. Furthermore, in the case of a method in which a recording medium formed of multi-layered recording films of different Curie temperature as described hereinabove, the number of layers is limited due to difficulty in fabrication process, and the dynamic range cannot be widened and the number of multiple values is limited. Furthermore, the increased film thickness of recording area due to increased number of layers results in reduced recording density in the plane direction due to heat diffusion in the direction of disc plane.
To secure the same recording density as that of the conventional method for multi-level recording by use of the above-mentioned elliptical recording pit, the area of a recording pit for the elliptical shape is reduced in comparison with that for a circular recording pit. Therefore the signal intensity of the single recording pit decreases inevitably and S/N becomes poor. Since the direction of the elliptical shape is detected by a plurality of detectors, the increased multi-level results in considerably reduced difference of pattern between each direction, and leads to a weak signal which is to be received by detectors. As a result, a large multi-level generates insufficient detection accuracy, and a large multi-level cannot be expected. As described hereinabove, the method in which multi-level data is recorded in a single recording pit by means of in-plane recording method is involved not only in a difficulty of high rate transfer but also in that of high density recording.
Accordingly, it is the object of the present invention to provide an optical recording method and an optical recording apparatus for realizing high density recording and high speed recording and an optical reading method and an optical reading apparatus for realizing high speed reading.
The inventors of the present invention were engaged in research and development of multi-level recording to solve the above-mentioned problems. As a result, it was found that a recording light having the spatial polarization distribution was irradiated onto an optical recording medium which exhibited photo-induced birefringence and then a reading light having an arbitrary uniform polarization direction was irradiated onto the optical recording medium, thereby the transmitted light having the polarization distribution corresponding to that of the recording light was obtained, and thus the present invention was accomplished.
In detail, in an optical recording method in accordance with the present invention, a recording light comprising a plurality of polarization distributions and the recording light is irradiated onto an optical recording medium to thereby record the recording light comprising the plurality of polarization distribution on the recording medium as the photo-induced birefringence distribution. More preferably, the recording light which has been subjected to Fourier transformation is irradiated onto the optical recording medium. The recording light is formed so as to have a two-dimensional polarization distribution formed correspondingly to the data information.
The optical recording medium contains at least one layer of optical recording material which exhibits photo-induced birefringence. The thickness of the optical recording material satisfies the relation xcex94nxc2x7d=(m+xc2xd)xc2x7xcex or the relation xcex94nxc2x7d=(m+xc2xc)xc2x7xcex, where m denotes an integer of 1 or larger, xcex denotes a wavelength of the reading light, and xcex94n denotes birefringence change induced by the light. A polymer or polymeric liquid crystal having an isomerizable side chain or a polymer in; which photoisomerizable molecules are dispersed is used as the optical recording material. The isomerizable group or molecule contains an azobenzene structure. The polymer or polymeric liquid crystal is at least polymerized products selected from a group of polyesters.
An optical recording apparatus in accordance with the present invention is provided with a light source for emitting a coherent light, a spatial light modulator for polarization modulating the light from the light source correspondingly to the data information to obtain a recording light, and a focusing optical system for irradiating the recording light onto an optical recording medium. The focusing optical system is preferably structured so that the recording light is subjected to Fourier transformation. The spatial light modulator is structured with a opto-electric conversion material having transparent electrodes formed on both sides thereof. For example, a liquid crystal is used as the opto-electric conversion material.
An optical recording apparatus in accordance with the present invention is provided with an optical recording head having a light source for emitting a coherent light, a spatial light modulator for polarization modulating the light from the light source correspondingly to the data information to obtain a recording light, and a focusing optical system for irradiating the recording light onto an optical recording medium, a head moving mechanism for moving the optical recording head in the radial direction of the optical recording medium, and a medium driving mechanism for driving the optical recording medium. The optical recording medium may be a disc-shaped optical recording medium, and the optical recording medium may be incorporated in the optical recording apparatus.
In an optical reading method in accordance with the present invention, a reading light having a uniform polarization distribution is generated, the reading light is irradiated onto an optical recording medium having a photo-induced birefringence, and the polarization distribution of the transmitted light or reflected light from the optical recording medium is read as the polarization distribution of a reproducing light. Preferably, the reading light which has been subjected to Fourier transformation is irradiated onto the optical recording medium. In this case, the transmitted light or reflected light is subjected to inverse Fourier transformation. The polarized reading light is a linearly polarized light having a certain arbitrary direction. The transmitted light or reflected light is split into two orthogonal polarization components, light intensities of both of the polarization components are compared with each other and calculated, and the comparison and calculation result is read.
An optical reading apparatus in accordance with the present invention is provided with a reading optical system for irradiating a reading light onto an optical recording medium where a recording light having the data information in the form of spatial polarization distribution is recorded in the form of photo-induced birefringence, and a photo-detector for detecting the polarization distribution of the transmitted light from the optical recording medium. Otherwise, an optical reading apparatus is provided with a reading optical system for irradiating a reading light onto an optical recording medium where a recording light having the data information in the form of spatial polarization distribution is recorded in the form of photo-induced birefringence, an optical element for receiving the reflected light from the optical recording medium, and a photo-detector for detecting the polarization distribution of the reflected light from the optical element.
The reading optical system has a spatial light modulator for forming a linearly polarized reading light having an arbitrary direction. The photo-detector is for detecting the polarization distribution of the transmitted light through the polarizing beam splitter and comprises two photo-detectors for separately detecting two orthogonal polarization components split by the polarizing beam splitter. The detected outputs of these respective two light detectors are compared and calculated by the comparison calculation unit and the reproducing polarization distribution is read.
By applying the above-mentioned methods and apparatus, the optical recording method, optical recording apparatus, optical reading method, and optical reading apparatus which are capable of high density recording and high speed recording, and high speed reading are obtained.